The present invention relates to heat treatment jigs for use in the manufacture of cathode-ray tubes, and in particular to a heat treatment jig for use in the baking process, the frit seal process, and the evacuation process, the jig employing a novel material for the support portion which supports the item to be heat treated.
During the production of a cathode-ray tube (hereinafter referred to as a CRT) various components require heat treatment, both before and after being assembled with other components. For example, a panel glass assembly such as the one shown in FIG. 1 will undergo heat treatment two times before being assembled with the envelope of the CRT. The panel glass assembly of FIG. 1 comprises a panel glass 1 having a seal 1a, and an inner surface 1b, the latter of which is coated with a fluorescent surface 2. Inside the panel glass 1 is housed a shadow mask 3, which is attached to the panel glass 1 with panel pins 4. Prior to and subsequent to the formation of the fluorescent surface 2 on the inner surface 1b, the panel glass assembly undergoes heat treatment at 400.degree.-500.degree. C. in a continuous kiln. This heat treatment of the panel glass assembly is referred to as the baking process.
FIG. 2 shows a panel glass assembly mounted on a heat treatment jig (in this case, a "baking jig") as it would appear during the baking process. The baking jig 5 comprises a base 6 and heat resistant, electrically insulating support members 7, atop which sits the panel glass assembly.
The success of the baking process and the quality of the completed CRT are greatly dependent upon the physical properties of the material used for the support members 7. If the support members 7 are made of an extremely hard material, they produce scratches, cracks, and other mars in the seal 1a which can result in breakage of the panel glass 1.
On the other hand, if the support members 7 are made of an excessively soft material, different but equally serious problems may occur. For example, if the support members 7 are too soft, they will wear down rapidly and require frequent replacement. More importantly, the dust produced by such wearing down may ruin the panel glass assembly if it enters inside the seal 1a. That is, such dust mars the insulating properties of the CRT, and if dust adheres to some location on the seal 1a, the high voltage existing during operation of the CRT may produce a breakdown in insulation at that location, making the CRT unusable.
At present, the most widely used materials for the support members 7 of baking jigs are asbestos tape, asbestos cement, and boron nitride, all of which have drawbacks. Asbestos tape does not produce scratches or cracks, but it easily produces dust. Asbestos cement, being harder than asbestos tape, does not produce dust, but it tends to scratch the seal 1a of the glass panel 1. Further, both asbestos tape and asbestos cement pose health hazards. Boron nitride does not mar the seal 1a, but it adheres to the seal 1a. After the baking process, the panel glass assembly is connected to the funnel of the CRT envelope using solder glass in the so-called frit seal process. However, at those locations on the seal 1a that boron nitride adheres to, solder glass will not adhere, resulting in a mechanically and eletrically weak seal between the panel glass assembly and the funnel, which may result in serious functional problems. Moreover, boron nitride is not only extremely expensive, it also wears down quite rapidly.
Asbestos tape, asbestos cement, and boron nitride are also used for the support members of heat treatment jigs used in the frit seal process and the evacuation process during the manufacture of CRT's. Here, these materials pose much the same problems as they do when used in baking jigs. Because of their use of these three materials, conventional heat treatment jigs are far from satisfactory.